package org.liuyuantao.thread.concurrent;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

/**
 * 信号灯,同时可以允许多少个并发执行,其他的线程只能等待
 * 单个的Semaphore可以实现类似线程的Synchronized。
 * 通常用于限制可以访问某些资源（物理或逻辑的）线程数目
 */
public class SemaphoreTest {
    public static void main(String[] args) {
        ExecutorService service = Executors.newCachedThreadPool();
        final Semaphore sp = new Semaphore(3);
        for (int i = 0; i < 10; i++) {
            Runnable runnable = () -> {
                try {
                    sp.acquire();
                } catch (InterruptedException e1) {
                    e1.printStackTrace();
                }
                System.out.println("线程" + Thread.currentThread().getName() +
                        "进入,当前已有" + (3 - sp.availablePermits()) + "个并发");
                try {
                    Thread.sleep((long) (Math.random() * 10000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("线程" + Thread.currentThread().getName() +
                        "即将离开");
                sp.release();
                //下面代码有时候执行不准确,因为其没有和上面的代码合成原子单元
                System.out.println("线程" + Thread.currentThread().getName() +
                        "已离开,当前已有" + (3 - sp.availablePermits()) + "个并发");
            };
            service.execute(runnable);
        }
    }

}
